Heterojunction bipolar transistors (HBTs) generally utilize ohmic contacts between emitter and/or base contacts and underlying epitaxial layers. Preferably, these ohmic contacts exhibit low resistance and significant thermal stability. For emitter contacts in gallium arsenide (GaAs) based HBTs, low resistance may be achieved by ensuring sufficient contact between the contact metal and an n+ indium gallium arsenide (InGaAs) layer. Thermal stability may be improved by controlling and/or limiting diffusion of the contact metal into the semiconductor epitaxial structure.
Traditionally it has been necessary to utilize different metallization schemes for emitter contacts, as compared to base contacts, to achieve low resistance and thermal stability for both types of contacts. This may require separate processing operations to form emitter contacts and base contacts.